Conventional refrigeration and air conditioning systems typically utilize a series of recirculating fluid loops to cool a space by transferring the heat from the space through the fluid loops and ultimately to a heat sink such as water or ambient outside air. A commercial air conditioning system, for instance, typically includes a water chiller having an evaporator at its low pressure side, a condenser at its high pressure side, a compressor to boost the pressure of refrigerant as its flows from the evaporator to the condenser and an expansion valve to meter refrigerant from the high pressure condenser to the low pressure evaporator.
In a first fluid loop, water passes through the chiller evaporator, where it is cooled in a heat exchange relationship with relatively cooler system refrigerant, before being directed to a location where it absorbs heat and is returned to the evaporator. In "flooded design" evaporators, the water in the chilled water loop flows through the tubes of the evaporator and liquid refrigerant surrounds the outside of the tubes. The cooler liquid refrigerant surrounding the tubes absorbs heat energy from the relatively warmer water, thereby chilling the water.
During the removal of heat energy from the warm water, the liquid refrigerant vaporizes. The vaporized refrigerant is pumped out of the evaporator by the compressor which compresses the refrigerant, raising its pressure and temperature. The high temperature refrigerant then flows to the system condenser where its heat is rejected, most typically, to water in a second fluid loop or directly to ambient air.
As the refrigerant is cooled in the condenser it changes state from a hot gas to a warm, relatively high pressure liquid which is metered through a pressure reducing expansion valve, to the evaporator. The expansion valve maintains the pressure differential between the high and low pressure sides of the refrigeration system.
The pressure of the refrigerant is controllably reduced as it passes through the expansion valve to ensure that the refrigerant will efficiently vaporize and absorb heat from the relatively warm water flowing through the evaporator. The cycle is completed, and ready to be repeated, when the liquid refrigerant flows, at reduced pressure, through the expansion valve to the evaporator.
The amount of liquid refrigerant introduced into the evaporator should be that amount which can "wet" the surface area of the tubes of the evaporator without having more or less liquid refrigerant in the evaporator than is needed for a particular cooling load. Accordingly, the expansion valve should be adjustable, on command, to control the amount of liquid refrigerant introduced into the evaporator. The expansion valve can also be used to shut off the flow of refrigerant through the chiller such as for purposes of isolating chiller components for maintenance.
Electric, rotary actuated, incremental valves suitable for use as expansion valves in refrigeration systems are typically comprised of two types. In both types, the valve is operated by a stepping motor which provides incremental rotary motion which is then translated to incremental linear motion to actuate a valve element.
In the first type of valve, the valve element is linearly driven against the valve seat to sealingly cover an aperture to prevent fluid flow therethrough. Alternatively, the valve element is driven linearly away from the seat to open the valve by incrementally opening the aperture to flow. The distance of the valve element from the valve seat is determinative of the flow area available through the valve, up to a maximum which is restricted only by the size of the aperture itself.
This type of rotary actuated, linearly driven expansion valve may stick due both to the friction between valve parts and the viscosity of contaminants collected upon the valve body. Overcoming this tendency to stick can require the use of oversized, more costly motors.
The ability to carefully control the system is decreased because the actuator may fail to overcome the sticking of the valve element, for one or more actuating pulses or steps of the motor, causing the valve element to be improperly positioned. The valve controller, having sent a specified number of pulses intended to actuate the valve to a desired opening, will in fact have actuated the valve to a smaller degree than calculated or desired. The system then indicates the need for further movement of the valve element and the controller begins to "hunt" for the appropriate valve setting.
In a second type of rotary expansion valve, the valve element is a member which is linearly driven normal to a flow aperture and the direction of flow of refrigerant through the valve. In this type of valve, the amount of flow is determined by the extent of the area of the aperture which is uncovered by the valve element.
In addition to the potential "hunting" problems described above, this second type of rotary actuated expansion valve is typically comprised of many relatively small parts which must be machined to close tolerances to prevent fluid leakage therethrough and to improve operating characteristics. Such close tolerance machining is often expensive and time consuming, as is the assembly of valves containing such parts.
Furthermore, this type of rotary actuated expansion valve often includes a relatively large number of elastomeric seals to prevent leakage past the valve element and oftentimes, one or more springs having a relatively large traverse distance, all of which are susceptible to wear and breakage. These items tend to substantially decrease the reliability of the valve.
It is an object of the present invention to provide a rotary actuated expansion of relatively rugged design and simple construction that avoids the problems inherent in stepper motor driven expansion valves having a linearly driven valve element.
It is a further object of the present invention to provide a rotary actuated expansion valve which is highly reliable yet relatively low in manufacturing and maintenance requirements and costs.
It is yet another object of the invention to provide such a valve which is capable of being mass produced.
It is yet another object of the invention to provide such a valve which is a low friction device.
It is yet another object of the present invention to provide such a valve as will appropriately respond to a controller input to permit smooth, pulse-free fluid flow through the valve.
It is yet another object of the invention to provide such a valve which is suitable specifically for such applications as an expansion valve in an air conditioning or other type of refrigeration system, particularly where such system includes a flooded evaporator.
These and other objects of the present invention will be apparent from the attached drawings and description of the preferred embodiment which follows.